The technical field of this invention is holography and, in particular, methods and devices for recording and projecting holographic stereograms for computer-aided design displays and other applications.
The growing speed and memory capacities of modern computers have made possible the computation and analysis of fully three-dimensional problems that could only be contemplated a few years ago. But the input and output of three-dimensional data continue to be substantial bottlenecks in the efficient use of such systems. For example, the design and evaluation of complex structures, such as homes, offices and automobiles, is increasingly being undertaken by computer, wherein the same data base can serve for both engineering and styling evaluation. Such structures can become understood from their two-dimensional renderings onto CRT screen surfaces by their designers, who spend hours studying the subject from many different viewpoints, but the rapid and accurate communication of a complex, three-dimensional shape to another human's mind requires its presentation in fully three-dimensional form.
Traditionally, presentations of three-dimensional designs have taken the form of physical models at various scales formed from wood, cardboard, or clay, which combine a visual and tactile verisimilitude that has an enduring appeal. However, much more rapidly rendered, visually solid representations are available directly from the computer's data base via various optical and electronic output modalities. Described herein is a new type of computer-graphic hologram called the "alcove" hologram that offers an important step toward a visual experience that rivals physical models in their realistic sense of solidity and shape.
The technologies for generating holograms with computers typically fall within one of three types. In the first case, the hologram's dark-and-light fringe patterns are computed by mathematically stimulating the light propagation and interference processes, and then recording those fringes as photographic output. See generally, W. H. Lee, "Computational Holography," in E. Wolf, ed., "Progress in Optics," Vol. XVI (North Holland Publ. Co., 1978) pp. 121-122, herein incorporated by reference. The fringes of a typical hologram are very closely spaced, perhaps 1500 cycles per millimeter (40,000 line pairs per inch), so that an enormous amount of calculation is required, and the recording of the output fringes is currently possible only with electron-beam lithography equipment ordinarily used to manufacture integrated circuitry. Such methods are beyond the practical reach of current technology.
The second approach is to record a patch of an off-axis Fresnel zone plate (a "diffraction lens," produced by interference) on a photographic plate for every resolvable point of the image. See generally, H. J. Caulfield, "Hologram Write and Method," U.S. Pat. No. 4,498,740 (issued Feb. 12, 1985), herein incorporated by reference. Several hours are required to write tens of thousands of patches, and the diffraction efficiency of the hologram decreases as the reciprocal of the number of patches overlapping at any point, so that the number of usable points is less than 10,000 in practice.
Thirdly, for visual applications, the resolution potential of true or fully computed holograms often is not required, as nothing beyond the resolution of the unaided eye will be needed. In any case, if the image is to be computer generated, its resolution is limited by the pixelation of the calculation. See generally, S. A. Benton, "Photographic Holography," in SPIE Proc. Vol. 391--"Optics in Entertainment" (1983), pp. 2-9, herein incorporated by reference. A series of perspective views, corresponding to the expected viewpoints of the audience, can be computed by conventional computer-graphic techniques. Each perspective view can then be projected with laser light onto a piece of high resolution film from the angle corresponding to its computed viewpoint, overlapped by a coherent "reference" beam to produce a holographic exposure that records the direction of the image light. After all the views have been recorded in this way, the hologram is processed and then illuminated so that each view is sent back out in the direction it was projected from, that is, toward its intended viewing location, so that a viewer moving from side to side sees a progression of views as though he or she were moving around an actual object.
If the images are accurately computed and registered, the resulting image looks like a solid 3-D object. Such a composite or synthetic hologram is termed a "holographic stereogram." It mimics the visual properties of a true hologram even though it lacks the information content and interferometric accuracy of a true hologram.
Holographic stereograms have proven capable of rendering very satisfying and solid looking computer graphic images. However, the best of them have required two optical steps and a correspondingly long time to produce and offer only a limited image projection and angle of view. There exists a need for better holographic methods and devices that can provide images that project strikingly, to within the viewer's grasp, and that offer a very wide angle of view. Further, if such images can be produced in a single optical step by a technique that can be extended to the rapid and automatic production of very large-size holograms, a long-felt need in the industry would be satisfied. The present invention addresses such needs and is specifically designed to be adaptable to a computer-graphic, 3-D hard copy peripheral device for computer-aided design systems.